Electrons, Waves and Photons Flashcards
State Ohm’s Law
Provided the temperature is constant, the current through an ohmic conductor is directly proportional to the potential difference across it (V=IR)
Electromotive force units
J C^-1
Resistance units
V A^-1
Energy units
V C
Charge units
A s
Define potential difference
energy per unit charge
transferred from electrical to other forms
What is meant by internal resistance.
-(some) energy is transferred into thermal energy /lost as heat in (driving charge through) the battery. It behaves as if it has an (internal) resistance
or
-there is a voltage drop across/decrease in voltage from the battery (when a current is drawn from it)
Define resistance.
-p.d./voltage (across component) divided by current (in it)
If two sources are coherent…
…they have the same wavelength and frequency and a fixed phase difference between them.
Define Intensity
energy per unit time/power per unit area (perpendicular to the direction of energy transfer)
State two properties which distinguish EM waves from other transverse waves.
- all travel at speed of light through a vacuum
- are oscillating E and B fields or are caused by accelerating charges
Malus’s Law
I= Io cos^2(feta)
What is a stationary wave?
A stationary wave is the superposition of two progressive waves with the same wavelength, moving in opposite directions.
How do stationary waves form?
- waves are reflected
- reflected waves interfere/superpose with the incident
- waves to produce nodes and antinodes
Define work function
the minimum energy required for an electron to escape from metal surface
Explain how the work function is related to the threshold frequency.
- a photon with less than the threshold frequency cannot cause electron emission
- so work function = h (threshold frequency)
Define power
Work done/energy transfer(red) per unit time
Define kilowatt-hour
- (a unit of) energy equal to 3.6 MJ
- 1 kW for 1 hr
State and explain how the current I in the circuit changes as the thermistor is heated.
- R of thermistor decreases as temperature increases
- supply V is constant/ total R is smaller
- current increases as V = IR
State with a reason how the voltmeter reading varies as the light incident on the LDR increases.
R of LDR decreases/current in circuit increases so V increases across fixed resistor
Explain the term e.m.f.
energy transferred from source/changed from some form to electrical energy per unit charge
Both electrons and photons can be considered as particles. State two differences between their properties.
- electrons have mass, photons have zero mass
- electrons have charge, photons are uncharged
- photons travel at speed of light
Explain what is meant by the de Broglie wavelength of an electron.
electron wavelength depends on its speed/momentum
Displacement- wave motion
distance moved from equilibrium of a point/particle on a wave
Amplitude- wave motion
maximum displacement (caused by wave motion)
Frequency- wave motion
number of wavelengths passing a point /vibrations at a point per unit time/second or produced by the wave source
Phase difference- wave motion
phase difference between two points on the same wave/waves of the same frequency, how far through the cycle one point is compared to the other
Microwaves
10^-4
Ultraviolet
10^-8
Gamma Rays
10^-12
Infra red light
10^-6
Explain why the resistance of the filament lamp is much larger at a higher voltage.
the resistivity/resistance of the (metal) filament increases with temperature
the larger the current in the filament the hotter it becomes
Emf (units)
J C^-1
Energy (units)
V C
What is meant by internal resistance.
- (some) energy is transferred into thermal energy /lost as heat in (driving charge through) the battery. It behaves as if it has an (internal) resistance
- there is a voltage drop across/decrease in voltage from the battery (when a current is drawn from it)
If two waves are coherent it must mean:
constant phase difference/relationship (between the waves) or always at π radians/180degrees
or because they are generated by the same source
In a series circuit with two resistors, state one electrical quantity which is the same for both resistors.
Current
In a parallel circuit with two resistors, state one electrical quantity which is the same for both resistors.
Potential difference.
Explain why the terminal p.d. is less than the e.m.f.
- any source has an internal resistance
- where energy is transferred into thermal energy / lost as heat.
What is meant by diffraction of a wave.
wavefronts/paths spread out after passing through a gap around an obstacle.
State two features of a stationary wave.
-energy is trapped in pockets/ where the shape or energy
-does not move along/energy is stored
-there are nodes/positions of zero amplitude
-incident wave is reflected (at the fixed end of the string) -and the reflected wave (or it) interferes/superposes with
the incident wave (to produce the stationary wave)
Mean drift velocity
the average displacement/distance travelled of the electrons along the wire per second
Name the charge carriers responsible for electric current in a metal and in an electrolyte.
- Electrons in a metal
- Ion in an electrolyte
Explain how you would determine experimentally the e.m.f and internal resistance of the charged cell. Include a circuit diagram with meters and a variable load.
1.cell across variable resistor R ammeter in series and voltmeter in parallel across R or cell
2.Take (set of) readings of V and I for different positions/values of the variable resistor
3.plot a graph of V against I
4.(find) y-intercept = E
5.(find) the gradient of the V against I graph which equals the internal resistance in magnitude
or 4 or 5 take one pair of values of V,I and substitute
into equation E = V + Ir to find r or E
Photoelectric effect
- Individual photons are absorbed by individual electrons ( in the metal surface)/ one to one interaction/AW
- Only photon with energy above the work function energy will cause photoelectron emission/idea of threshold frequency 3.Photon energy is proportional to frequency
- (therefore) blue photons with higher f/shorter λ will cause photoemission but red photons will not.
- hf – φ = KEmax is the equation resulting from conservation of energy or resulting from the meaning of each term
- A wave model does not explain instantaneous emission
The amplitude if transmitted waves is doubled, what happens to intensity and position of maxima.
- intensity increases by factor of 4
- position unchanged
The separation between the transmitters is halved, what happens to intensity and position of maxima.
- intensity unchanged
- distance apart of maxima is doubled
The phase of transmitter A is reversed so that there is now a phase difference of 180 degrees between the waves A and B, what happens to intensity and position of maxima.
- intensity unchanged
- maxima move to positions of minima (and vice versa)
Explain what energy levels are and how they can be used to explain the emission of photon from atoms.
- energy levels explanation: electrons have discrete energies in atom
- each photon produced by electron moving between levels
- photon energy equal to energy difference between levels
- electron loses energy/making transition in correct direction
State the condition necessary for electrons to produce observable diffraction when passing through matter, e.g. a thin sheet of graphite in an evacuated chamber.
- wavelength of electrons
- must be comparable/of the order of magnitude of the atomic spacing
The charge carriers moving through the electrolyte to the positive terminal of the battery.
positive ions/ cations
The charge carriers moving through the wires to the positive terminal of the battery.
electrons
Explain why the p.d. across the motor is not the same as the e.m.f of the car battery.
the battery has an internal resistance
some of the emf is across the (internal) resistance (leaving a smaller p.d. across motor)
Explain how and why the resistance of the headlamp filament varies with the current passing through it.
- The current heats the filament
- The resistance/resistivity (of the metal filament) increases (with temperature).
Kirchoff’s 1st Law and the quantity conserved.
(sum of/total) current into a junction equals the (sum of/total) current out
-conservation of charge
Kirchoff’s 2nd Law and the quantity conserved.
- (sum of) e.m.f.s = (sum /total of) p.d.s/sum of voltages in/around a (closed) loop (in a circuit)
- energy is conserved
Describe and explain the photoelectric effect.
-a photon is absorbed by an electron (in a metal surface);
-causing electron to be emitted (from surface).
-Energy is conserved (in the interaction).
-Only photons with energy/frequency above the work function energy/threshold frequency will cause emission –Reference to Einstein’s photoelectric energy equation
-(energy of photon) = (work function of metal) + (maximum
possible kinetic energy of emitted electron)
-work function energy is the minimum energy to release an electron from the surface
-Number of electrons emitted also depends on light intensity
-Emission is instantaneous
Explain what is meant by the de broglie wavelength of an electron.
Electrons are observed to behave as waves/show wavelike properties
where the electron wavelength depends on its speed/momentum
Continous spectrum.
all wavelengths are present in the radiation.
